In evolutionary biology and evolutionary psychology, the Trivers–Willard hypothesis, formally proposed by Robert Trivers and Dan Willard, suggests that female mammals are able to adjust offspring sex ratio in response to their maternal condition. For example, it may predict greater parental investment in males by parents in "good conditions" and greater investment in females by parents in "poor conditions" (relative to parents in good condition). The reasoning for this prediction is as follows: Assume that parents have information on the sex of their offspring and can influence their survival differentially. While pressures exist to maintain sex ratios at 50%, evolution will favor local deviations from this if one sex has a likely greater reproductive payoff than is usual.
Trivers and Willard also identified a circumstance in which reproducing individuals might experience deviations from expected offspring reproductive value—namely, varying maternal condition. In polygynous species males may mate with multiple females and low-condition males will achieve fewer or no matings. Parents in relatively good condition would then be under selection for mutations causing production and investment in sons (rather than daughters), because of the increased chance of mating experienced by these good-condition sons. Mating with multiple females conveys a large reproductive benefit, whereas daughters could translate their condition into only smaller benefits. An opposite prediction holds for poor-condition parents—selection will favor production and investment in daughters, so long as daughters are likely to be mated, while sons in poor condition are likely to be out-competed by other males and end up with zero mates (i.e., those sons will be a reproductive dead end).
The hypothesis was used to explain why, for example, Red Deer mothers would produce more sons when they are in good condition, and more daughters when in poor condition. In polyandrous species where some females mate with multiple males (and others get no matings) and males mate with one/few females (i.e., "sex-role reversed" species), these predictions from the Trivers–Willard hypothesis are reversed: parents in good condition will invest in daughters in order to have a daughter that can out-compete other females to attract multiple males, whereas parents in poor condition will avoid investing in daughters who are likely to get out-competed and will instead invest in sons in order to gain at least some grandchildren.
"Condition" can be assessed in multiple ways, including body size, parasite loads, or dominance, which has also been shown in macaques (Macaca sylvanus) to affect the sex of offspring, with dominant females giving birth to more sons and non-dominant females giving birth to more daughters. Consequently, high-ranking females give birth to a higher proportion of males than those who are low-ranking.
The Trivers–Willard hypothesis rests on certain assumptions:
- Parental condition is associated with offspring condition;
- Differences in offspring condition will persist into adulthood;
- Being in condition differentially affects the mating success of one sex more than it does the other.
Evolutionary biologists predict a Trivers–Willard effect where these conditions hold, and no effect when these conditions do not hold. In polygynous species where some males have multiple mates and others have none (i.e., greater variance in mating success among males than females), being in good condition affects males more than females. This is reversed in polyandrous species, and possibly in species where condition is based on social status and males disperse.
In their original paper, Trivers and Willard were not yet aware of the biochemical mechanism for the occurrence of biased sex ratios. Eventually, however, Melissa Larson et al. (2001) proposed that a high level of circulating glucose in the mother's bloodstream may favor the survival of male blastocysts. This conclusion is based on the observed male-skewed survival rates (to expanded blastocyst stages) when bovine blastocysts were exposed to heightened levels of glucose. As blood glucose levels are highly correlated with access to high-quality food, blood glucose level may serve as a proxy for "maternal condition". Thus, heightened glucose functions as one possible biochemical mechanism for observed Trivers–Willard effects.
Wild and West published a paper describing a mathematical model built on the Trivers–Willard hypothesis that allows precise predictions of alterations in sex-ratio under different circumstances.
The Trivers–Willard hypothesis has been applied to resource differences among individuals in a society as well as to resource differences among societies. Empirical evidence is mixed with higher support in better studies according to Cronk in a 2007 review. One example, in a 1997 study, of a group with a preference for females was Romani in Hungary, a low-status group. They "had a female-biased sex ratio at birth, were more likely to abort a child after having had one or more daughters, nursed their daughters longer, and sent their daughters to school for longer".
- Trivers, R. L.; Willard, D. E. (1973). "Natural selection of parental ability to vary the sex ratio of offspring". Science. 179 (4068): 90–92. doi:10.1126/science.179.4068.90. PMID 4682135.
- Kuesterl, A. Paul; et al. (1992). "Maternal rank affects reproductive success of male Barbary macaques (Macaca sylvanus): evidence from DNA fingerprinting". Behavioral Ecology and Sociobiology. 30 (5): 337–341. doi:10.1007/BF00170600.
- Larson, M.; et al. (2001). "Sexual Dimorphism among Bovine Embryos in Their Ability to Make the Transition to Expanded Blastocyst and in the Expression of the Signaling Molecule IFN-τ". Proc. Natl. Acad. Sci. USA. 98 (17): 9677–9682. doi:10.1073/pnas.171305398.
- Lieberman, Leslie (2003). "Dietary, Evolutionary and Modernizing Influences on the Prevalence of Type 2 Diabetes". Annual Review of Nutrition. 23: 345–377. doi:10.1146/annurev.nutr.23.011702.073212.
- Wild, Geoff; West, Stuart A. (2007). "A Sex Allocation Theory for Vertebrates: Combining Local Resource Competition and Condition‐Dependent Allocation". The American Naturalist. 170 (5): E112–E128. doi:10.1086/522057. ISSN 0003-0147. PMID 17926288.
- Cronk, L. (2007). "Boy or girl: Gender preferences from a Darwinian point of view". Reproductive BioMedicine Online. 15: 23–32. doi:10.1016/S1472-6483(10)60546-9. PMID 18088517.